Industrial loads connected to electric ac networks, in particular arc furnaces but also, for example, rolling mills, influence the voltage of the ac network, above all by a greatly varying consumption of reactive power during operation. Especially when the variations in power consumption lie within a frequency interval of typically 0-20 Hz, the corresponding voltage variations, so-called flicker, are observable and disturbing to the human eye in case of electric lighting supplied by the ac network.
It is known, in connection with industrial loads of the above-mentioned kind, to connect in parallel therewith, that is, in a shunt connection to the ac network, static compensators for compensation of the reactive power consumption of the load. One type of such a compensator usually comprises a device for generating reactive power as well as a device for controllable consumption of reactive power, known within the art as a thyristor-controlled reactor (TCR). The device for generation of reactive power usually comprises one or more mutually parallel-connected filters, each essentially comprising an inductive element in series-connection with capacitive elements. The filters are tuned to chosen multiples of the nominal frequency of the ac network, for example to the 3rd, 4th and 5th tones, sometimes even to the 2nd and 7th tones. The device for controllable consumption of reactive power comprises an inductive element, an inductor, in series-connection with a controllable semiconductor valve. The controllable semiconductor valve comprises two controllable semiconductors, usually thyristors, in anti-parallel connection. By phase-angle control of the semiconductors, that is by controlling their turn-on angle relative to the phase position of the voltage of the ac network, the susceptance of the device and hence its consumption of reactive power can be controlled.
For a general description of thyristor-controlled reactors, reference is made to Ake Ekstrom: High Power Electronics HVDC and SVC, Stockholm June 1990, in particular to pages 1-32 to 1-33 and 10-8 to 10-12.
The compensator generates a reactive power equal to that generated by the device for generation of reactive power, reduced by the consumption in the thyristor-controlled reactor. By determining the instantaneous consumption of reactive power by the load and then controlling the power consumption of the thyristor-controlled reactor to such a value that, together with the consumption of the load, it corresponds to the reactive power generated by the device for generation of reactive power, the reactive power exchange with the ac network thereby becoming zero.
European patent EP 0 260 504 describes a circuit for compensation of reactive power comprising a compensator and a load of the above-mentioned kind. In addition thereto, this circuit comprises a self-commutated converter, controlled in pulse-width modulation in dependence on control signals generated in a control member, and connected to the ac network in parallel connection with the load and the thyristor-controlled reactor. The converter supplies to the ac network a reactive current for compensation of the active and reactive power consumed/generated by the load and the thyristor-controlled reactor. In an orthogonal two-phase system, in dependence on sensed three-phase currents and three-phase voltages, the control member calculates instantaneous values of the active and reactive power consumed/generated by the load and the thyristor-controlled reactor together.
The above-mentioned specification states that the voltage variations in the ac network are substantially determined by variations in the reactive power consumption of the load and that the voltage variations in dependence on its active power consumption may be neglected. The control signals to the converter are therefore formed only in dependence on variations in the consumption of reactive power by the load.
The method used in the above-mentioned patent for determining the instantaneous active and reactive power of the load in an orthogonal two-phase system is also applicable for control of a thyristor-controlled reactor. However, in this connection it has proved to be difficult, and in certain cases impossible, with the method for forming a control signal as stated in the above-mentioned patent, to meet the increasingly more stringent demands on allowable disturbances imposed by the operators of the ac networks.